Remove redundant empty check in poll()

Doing the check twice, both inside the critical section, didn't really
make much sense.

Test: Treehugger
Change-Id: I970394823c34d8eb20406978ecb5369ec0f0d713
diff --git a/ojluni/src/main/java/java/lang/ref/ReferenceQueue.java b/ojluni/src/main/java/java/lang/ref/ReferenceQueue.java
index 0d70709..3f16435 100644
--- a/ojluni/src/main/java/java/lang/ref/ReferenceQueue.java
+++ b/ojluni/src/main/java/java/lang/ref/ReferenceQueue.java
@@ -169,9 +169,6 @@
      */
     public Reference<? extends T> poll() {
         synchronized (lock) {
-            if (head == null)
-                return null;
-
             return reallyPollLocked();
         }
     }